1. Field of the Invention
The present invention relates to an exposure apparatus and a device manufacturing method using the same.
2. Description of the Related Art
An exposure apparatus which projects the pattern of an original (also called a reticle or mask) onto a substrate (wafer) via a projection optical system to expose the substrate to light is used in lithography for manufacturing a device such as a semiconductor device. To manufacture large-scale integrated devices, it is necessary to minimize the aberration of the projection optical system.
However, when the projection optical system receives an exposure load (heat) upon substrate exposure, its optical elements deform. This may result in an image shift on the optical axis, for example, astigmatism on the entire surface of the projection optical system. The astigmatism means an aberration in which light which emerges from one off-axis point diverges into two orthogonal light beams shifted forward and backward from each other without converging to one point after passing through the lens. If astigmatism is present, the hole shape changes due to defocusing. To solve this problem, an astigmatism correction mechanism has been proposed.
FIG. 1 is a graph illustrating the relationship between the C5 and C12 terms of the Zernike polynomials when the astigmatism correction mechanism performs aberration correction. The abscissa indicates the amount of change in the C5 term (corresponding to a low-order component of astigmatism), and the ordinate indicates the amount of change in the C12 term (corresponding to a high-order component of astigmatism). The C5 and C12 terms are mutually dependent such that the C12 term is determined if the C5 term is given, while the C5 term is determined if the C12 term is given.
If the ratio between a low-order component and high-order component of the aberration amount to be corrected is close to that of the sensitivity of the correction mechanism, it is possible to correct the aberration of the projection optical system with high accuracy. On the other hand, if these ratios are largely different from each other, the high-order aberration component deteriorates when the low-order aberration component is corrected with high accuracy, while the low-order aberration component deteriorates when the high-order aberration component is corrected with high accuracy. The ratio between the low-order component and high-order component of astigmatism generated by exposure heat changes depending on the illumination conditions used. Hence, one astigmatism correction mechanism alone can hardly correct astigmatisms, which are generated under all the illumination conditions, with high accuracy.